1. Field of the Invention
The invention relates generally to the field of lens systems, and specifically to the field of securing lenses within a lens system.
2. Background Information
Commonly assigned U.S. Pat. No. 5,557,474, which is hereby incorporated by reference, describes a passive thermal compensation mechanism that compensates for positional shifts between lenses in a lens group that occur due to thermal changes. This compensation can be useful, for example, to maintain as focal point of the lens group at a predetermined position through a temperature change. For example, the mechanism can be used within the optical path of a dual field of view missile seeker to permit both wide field of view (WFOV) and near field of view (NFOV) optical paths to remain parfocalized over a significant temperature range.
In particular, the '474 patent discloses a lens system for mounting lenses. As a temperature of the lens system changes, spacers in the lens system provide a displacement along the lens axis that compensates for radial expansion or contraction of the lenses along a radial plane. For example, where one or more of the spacers has a smaller thermal coefficient of expansion than the other ones of the spacers, the lenses will move apart as the temperature increases.
The '474 patent further discloses a biasing mechanism such as a load spring, elastic material or elastic member. The biasing mechanism ensures that the spacers in the lens system are abutting at all times and helps keep the lenses from moving or vibrating.
These principles are illustrated, for example, in FIGS. 1-3 of the present application. At room temperature, the optical system is adjusted so that both narrow field of view (NFOV) and wide field of view (WFOV) images are coincident (parfocalized). Thereafter, as temperature changes, the thermal effects on the optical materials and mechanical components would cause the NFOV and WFOV images to separate absent thermal compensation. As an example, it has been determined for a specific system in FIGS. 1, 3 and 3, that in order to adjust for defocus due to thermal changes, the air space between a first lens 1 and a second lens 2 must be varied at a rate of minus 0.007 inch per change of 30.degree. C. along the optical axis 20. This amount of positional shift accommodates various thermal effects including changes in the index of refraction, the lens shape and the relative dimensions of the lens and of various mechanical components.
The required air space change can be precisely accomplished by (1) controlling the contact slope angle of each space or interface, (2) determining the number of interfaces, and (3) properly selecting materials having different coefficients of thermal expansion (CTE).
Between lenses 10a and 10b, as shown in FIG. 1, are a series of annular spacers, each spacer having at least one angled side surface. The first, third and fifth spacers 11, 13 and 15, as shown in FIG. 1, are made of a material having a specific CTE. The even spacers 12 and 14 are made of a material having a different CTE. If a negative spacing change (wherein the spaced apart objects get closer as temperature increases) is desired, the odd annular rings 11, 13 and 15 of FIG. 1, should have a smaller CTE value than the matching even annular rings 12 and 14 of FIG. 1.
For instance, if a negative spacing change is desired, the odd spacers 11, 13 and 15 may be manufactured from a stainless steel alloy having a CTE of 5.5.times.10.sup.-6 /.degree. F. The even spacers 12 and 14 may be manufactured by ultrahigh molecular weight polyethylene having a CTE of 78.0.times.10.sup.-6 /.degree. F. This more than tenfold difference in the CTEs leads to selective control of the relative spacing between the lenses.
As the temperature rises, all of the spacers, as well as all of the mechanical elements of the system, expand. However, the even spacers 12 and 14 expand considerably faster than the odd spacers 11, 13 and 15. Being annular rings, the even spacers 12 and 14 move outward relative to the central axis of the optical system faster than the odd spacers 11, 13 and 15. To prevent gaps forming between the spacers as a temperature of the system changes, and to ensure that the air space between the lens elements 10a and 10b changes appropriately with the temperature, the lenses and rings are biased together so that the spacers in the lens elements are abutting at all times. A biasing mechanism 16 such as a load spring or an elastic material is used.
To package the lens system, a two part mount 17 and 18 is used. The two-part mount consists of a first mount or male member 18 through which a bore is formed for holding the lenses 10a and 10b, and the even and odd spacers 11-15. A second mount or female member 17 is screwed over the first mount member 18 to maintain the biasing mechanism 16 against the first lens element 10a to ensure that the lenses 10a and 10b are not free to move or vibrate, while also maintaining the abutting relationship of the elements.
If a positive dimensional change is desired, i.e., the lenses 10a and 10b move apart with increased temperature, one need only select materials wherein the even spacers 12 and 14 have a lower coefficient of thermal expansion than the odd spacers 11, 13 and 15.
As shown in FIG. 2, the number of interfaces n and the relative angle .phi. there between can be utilized to control the degree of change in the spacing between the two objects or lenses 10a and 10b. Specifically, the smaller the angle .phi. between the radial plane 19 and the interface surface, the smaller the degree of change in the spacing. The larger the angle .phi., of course, the greater the change in the spacing between the two objects 10a and 10b. However, too steep an angle .phi. might result in the interfaced surfaces locking together due to their relative coefficients of friction or Brinelling wherein the surface texture of one of the harder material punches into the surface of the softer material. To avoid the necessity of using too steep an angle, one might (a) properly select a greater number n of interfaces, (b) increase the contact radius R or (c) choose other materials having suitable coefficients of thermal expansion.
The interface contact angle of each of the rings may be determined by the following equation. ##EQU1## where: .phi.=contact angle
n=number of interfaces PA1 R=contact radius PA1 S=required airspace change over thermal change PA1 Ld=axial expansion of all spacers PA1 Cx=coefficient of thermal expansion of each material, wherein "x" is a number to identify the respective materials. PA1 Td=thermal change
With reference to FIG. 2, by solving this equation, a contact angle with 43.degree. at each of the four interfaces will yield minus 0.007 inch axial movement between the first lens 10a and the second lens 10b for a 30.degree. C. temperature change.
All spacer components 11-15 can be housed within a volume having a 1.2 inch outside diameter, a 1 inch inside diameter and a 0.25 inch length typical to the application shown. By selecting mixtures of contact angles .phi., a number of interfaces n, contact radii R and materials having specific CTEs for a given thermal range, a predetermined air space change rate with respect to temperature can be achieved.
With respect to FIG. 3, the system of FIGS. 1 and 2 is positioned inside a forward looking infrared system. As illustrated, a dome 30 protects the optical system, the first lens group 31 of which includes an embodiment of the present invention, as described above. This first lens group 31 is held by a spider, or four radially spaced armatures 32 which position the first lens group 31 from a second lens group 33 which receives the light from the first lens group. As illustrated, the lens groups are focal and include two focal planes, the common focal plane 34 pf the narrow field of view (NFOV) and wide field of view (WFOV) being illustrated in FIG. 3.
The NFOV image projects through the dome 30 onto the reflective surface 35 and reflects off the back surface of the second lens 10b of the first lens group 31 to form an image on the common focal plane 34. The WFOV image refractively travels through the first lens group 31 to focus on the common focal plane 34. The light, projecting through the common focal plane 34, then projects back through the second lens group 33 to a detector, not illustrated.
However, in order to prevent over-stressing the spacers or athermal rings, to allow the thermal compensation mechanism to function smoothly and reliably, and to reduce an overall bulk of the thermal compensation mechanism, a biasing mechanism is needed that provides a relatively constant spring load and requires only a small amount of volume.